In general, press-forming of thin plates is roughly classified into the three of bending forming, stretch forming and drawing. The bending forming is a method of forming the blank by bending it by using the die and the punch without clamping the margin of the blank. In contrast thereto, the stretch forming and the drawing are methods of forming the blank by pressing the punch against a forming region located at the center of the blank in a state of clamping the margin of the blank by the die and the blank holder.
FIG. 13 is an explanatory diagram showing the stretch forming. As shown in FIG. 13, in the stretch forming, a punch 4 is relatively pressed into a die 2 in a state of clamping mainly a clamp target part 1a provided on a margin of a blank 1 by trapezoidal beads 2a and 3a as one form of lock beads which are provided on/in the die 2 and a blank holder 3. Accordingly, a stretch-formed product is formed by stretching the blank 1 such that the margin of the blank 1 does not substantially flow (move) toward a forming region 1b of the blank 1, which corresponds to a product part. It is general that, for example, in automobile components, large-sized components having comparatively simple shapes such as a door outer panel, a hood outer panel, and a roof panel are manufactured by the stretch forming.
In contrast thereto, in the drawing, the punch is relatively pressed into the die in a state of clamping mainly the clamp target part provided on the margin of the blank by draw beads which are provided on/in the die and the blank holder. In the drawing, the amount of the blank which flows from the margin of the blank toward the forming region of the blank which corresponds to the product part is properly controlled per component by the draw beads during forming. Thereby, formability is controlled such that cracks, wrinkles and so forth are not generated in/on the product. It is general that a component which has a comparatively complicated shape, such as, for example, a side panel outer in the automobile components is manufactured by the drawing.
Both of the lock beads used in the stretch forming and the draw beads used in the drawing are the ones for adjusting a tensile force to be loaded on the blank such that shape defects such as the cracks and wrinkles and excessive surface deflection are not generated in the forming region (the product part) of the blank like this. However, while the stretch forming is the one for causing the blank not to flow from the margin of the blank into the forming region, the drawing is the one for causing the blank to flow from the margin of the blank into the forming region. Accordingly, the lock beads used in the stretch forming are different from the draw beads used in the drawing for controlling the inflow of the blank in the point that flowing of the blank from the margin of the blank into the forming region is substantially eliminated.
Hitherto, as the beads which are provided in/on the die and the blank holder and clamp the blank margin in order to eliminate flowing of the blank into the forming region, such trapezoidal beads as shown in FIG. 13 are generally known. The trapezoidal beads are substantially trapezoidal in section and clamp the blank 1 such that the blank 1 does not flow into it by deformation resistance of bending and unbending deformation of a trapezoidal corner part and frictional resistance caused by contact of the beads 2a and 3a each provided in/on the die 2 and the blank holder 3 with the blank 1.
FIG. 14 is an explanatory diagram showing one example of the blank 1 which has the clamp target part 1a to be clamped by the beads and the forming region 1b corresponding to the product part (a rear outer panel) of the blank 1 and is used in the stretch forming. FIG. 14 is the example of the blank 1 in a case of manufacturing two rear door outer panels from one blank 1.
As exemplified in FIG. 14, an outer peripheral part 1c of the clamp target part 1a of the blank 1 is cut off along a trim line 1d and is cut down together with the clamp target part 1a. Accordingly, if the outer peripheral part 1c and the clamp target part 1a can be set as small as possible after having ensured a blank clamping force which is required for the stretch forming, the entire size of the blank 1 will become small as much. Thereby, the material yield in the stretch forming is improved. In particular, since the stretch forming is used for forming the comparatively large-sized components as described above, an effect of reducing amount of used amount of material, that is, of improving the material yield owing to miniaturization of the blank 1 is large.
FIG. 15 to FIG. 17 are explanatory diagrams showing states of the vicinity of the trapezoidal bead 2a in a case of performing the stretch forming by using a die having the general trapezoidal beads 2a and 3a. FIG. 15 is perspective views showing the states before and after clamping the blank 1 by the die 2 and the blank holder 3. FIG. 16 is a sectional view showing the vicinity of the trapezoidal beads 2a and 3a including the clamp target part 1a, the outer peripheral part 1c and the trim line 1d of the blank 1. FIG. 17 is a top view showing the vicinity of the trapezoidal beads 2a and 3a including the clamp target part 1a, the outer peripheral part 1c and the trim line 1d of the blank 1. Incidentally, in FIG. 15 to FIG. 17, although a boundary between the inside and outside of the product is shown by a broken line, a region of a length L1 is a bead corresponding part of the blank 1 to be clamped by the beads 2a and 3a and is a part to be generally discarded.
The trapezoidal beads 2a and 3a are adapted to obtain a blank clamping force which is sufficient for the stretch forming by causing the bending and unbending deformation resistance of four trapezoidal corner parts 2a-1, 2a-2, 3a-1, and 3a-2 to generate. In mass forming, it is necessary to make the length of a sectional straight side part 2a-3 long to some extent in order not to destroy the trapezoidal bead 2a and in order to cause the bending and unbending deformation resistance of the trapezoidal corner parts 2a-1 and 2a-2 to be generated independently in the respective corner parts. Accordingly, the pressing length L1 of the trapezoidal beads 2a and 3a in a direction orthogonal to the trim line 1d becomes inevitably long. Therefore, it is difficult to shorten the pressing length L1 and to miniaturize the blank 1 with the trapezoidal beads 2a and 3a. 
In Patent Literature 1, there is disclosed a drawing method of, in a drawing apparatus which is provided with a die, a punch and a blank holder, drawing a material in a state of holding the material by the blank holder and a facing die in which draw beads having continuous bead parts that are non-parallel to a line parallel to a drawing profile of the material are formed on wrinkle pressing surfaces.
In Patent Literature 2, there is disclosed a drawing method of, in a drawing apparatus which is provided with a die, a punch and a blank holder, providing a movable die face which configures a die face part and is movable relative to a die body and a movable blank holder which faces the movable die face and is movable relative to a blank holder body, making the movable die face and the movable blank holder freely advance and retreat from the outside to the inside toward a forming depressed part in the die, driving the movable die face and the movable blank holder from the outside to the inside in association with pushing of the blank and thereby performing drawing in high yield such that a shock line does not enter a product part.
In Patent Literature 3, there is disclosed a press die device which performs drawing and stretching by forming a bead on one die and forming a bead containing part for containing the bead in a part facing the bead on another die, wherein step parts individual protruding heights of which are gradually reduced laterally are provided on a leading end of the bead, stepped depressed parts which correspond to the step parts are provided in the bead containing part, unevenness which corresponds to the step parts in a case where a blank material is nipped and held between the both is formed on an edge of the blank material and thereby generation of wrinkles can be prevented even in a case of being applied to a press machine having a low load capacity.
In Patent Literature 4, there is disclosed a bead to be provided on a wrinkle pressing surface of a draw die, the draw bead being configured by a vertical wall part and a corrugated part which is formed to be consecutive to the vertical wall part and is corrugated in section.